Optical discs are one type of data storage medium used to store a wide variety of digitally encoded data. Popular optical disc formats include compact discs (such as audio CD, CD-ROM, CD-R, CD-R/W) and digital versatile discs (such as DVD-audio, DVD-video, DVD-RAM, DVD-R, DVD-RW, DVD-ROM). Optical discs are usually portable in nature and can be played in a variety of settings including personal computers, car audio players, home theater systems, handheld personal data/entertainment devices, etc.
A typical optical disc comprises a circular disc having one or more recording layers of light reflective material embedded in a refractive substrate. Each recording layer is disposed along a plane substantially normal to an axis about which the disc is rotated and stores data in the form of localized pits and lands along a continuously extending spiral track. A data transducing head uses a laser or similar light source to output a readback signal based on the different reflectivities of the pit and land areas. Decoding circuitry decodes the user data for output by the appropriate playback device.
During readback, an optical disc typically provides main channel (user) data, control (subcode) channel data, and error detection/correction (EDC) channel data. Main channel data comprise the desired user data stored on the disc (audio, video, computer software, etc.) in fixed-size user data blocks (sectors). Control channel data comprise sector header, timing and other types of control information to facilitate playback of the main channel data. EDC channel data indicate the extent to which EDC techniques (parity bits, Reed-Solomon error correction codes, etc.) have been employed to correct detected errors in the main channel and control channel data.
The user data portion of an optical disc can be readily retrieved using various reading devices and stored onto other storage devices such as computer hard discs, floppy discs and recordable optical discs. Optical disc recording devices will accept this user data portion and add the additional address codes, synchronization data, error detection and correction codes, modulation data etc. using built-in per-programmed encoder circuits. This process is sometimes referred to as digital extraction or ripping.
Another increasingly common methodology for copying an existing disc is to use what is sometimes referred to as analog duplication. In this case the original disc is continuously read from lead-in to lead-out to generate a readback signal that is then used to sequentially, directly record the same pit and land sequence on a second, duplicate disc. The duplicate disc thus nominally comprises a bit-for-bit copy of the original and includes all of the errors and copy protection bits as they appear on the original.
Due to the continued world-wide interest in the types of data available on optical discs and other types of media, and the relative ease with which unauthorized duplicate copies can be made, there remains a continued need for improvements in the art for medium authentication schemes that are easily implemented and difficult to detect and defeat. It is to such improvements that the present invention is directed.